Cardiovascular disease, including atherosclerosis, is a leading cause of death in the U.S. A number of methods and devices for treating coronary heart disease have been developed, some of which are specifically designed to treat the complications resulting from atherosclerosis and other forms of coronary arterial narrowing.
One method for treating atherosclerosis and other forms of coronary narrowing is percutaneous transluminal coronary angioplasty, hereinafter referred to as “angioplasty” or “PTCA”. Many heart disease patients undergo angioplasty, some repeatedly.
The objective of angioplasty is to enlarge the lumen of the affected coronary artery by radial hydraulic expansion. This is generally accomplished by inflating a balloon within the narrowed lumen of the affected artery. Radial expansion of the coronary artery may occur in several different dimensions, and is related to the nature of the plaque. Soft, fatty plaque deposits are flattened by the balloon, while hardened deposits are cracked and split to enlarge the lumen. The wall of the artery itself may also be stretched as the balloon is inflated.
With simple angioplasty, the balloon is threaded through the artery with a catheter and inflated at the place where the blood vessel is blocked. After the procedure, the balloon is removed. Following simple angioplasty alone, arteries frequently close up again or re-narrow. This narrowing is known as restenosis.
To reduce the risk of restenosis, a stent may be inserted during angioplasty. The stent may be used to maintain the internal lumen of the artery after the balloon is removed. The use of a stent may significantly reduce the risk of restenosis. The stent is designed to support plaque damaged arterial walls after a blockage has been removed.
Typically, if restenosis occurs with a stent, the physician may insert highly radioactive pellets into the artery to help prevent further clogging. This radiation therapy can halve the risk of restenosis but presents all the risks associated with radiation therapy.
Blood vessel wall injury when the stent is implanted is one cause of restenosis. The area around the stent becomes inflamed and new cells form scar tissue. The arterial walls may become so thick in some instances that they protrude into the mesh of the stent. In such cases, a further angioplasty may be performed, and a new stent may be placed inside the existing one. If restenosis continues, the eventual alternative may then be bypass surgery.
Alternatively, a coated stent may be inserted during the angioplasty. Such a coated stent may eliminate the need for repeat angioplasties and could spare some patients the trauma, risk and prolonged recovery associated with heart bypass surgery.
The stent may be coated, for example, with rapamycin analogs or rapamycin derivatives. This drug is used to prevent organ rejection in kidney transplants. It stops new cells from forming without impairing the healing of the vessel. It also dampens inflammation and has antibiotic properties.
In clinical studies, patients who have received coated stents exhibited a substantially reduced re-narrowing and re-blockage of arteries.
However, because the coating of the stent comprises a therapeutic drug, coated stents present problems associated with drug administration. For example, for a drug to be administered effectively, the integrity of the drug's effective dosage must be maintained. Additionally, contamination of the drug should be avoided. Moreover, certain drugs require controlled conditions to maintain efficacy, including exposure to air, moisture, light etc.
Currently, stents may be protected with a sheath that closely surrounds the stent. With a coated stent, this protective sheath may damage the coating while the sheath is being placed on, or removed from, the stent. If the sheath is too tight, the coating may stick to the sheath rather than the stent. If the sheath is removed improperly, some of the coating may also be removed. In any of these cases, the dosage of the drug will be reduced.
In addition, stents may be sterilized or otherwise treated prior to deployment. Such treatments may also damage the coating.
It would be desirable therefore to provide a protective assembly for stents, coated stents, angioplasty balloons, and other medical devices used to treat restenosis that overcomes the above limitations.